Silica gel chromatography is a universal purification technique in organic synthesis, and perhaps one of the most practiced. Accordingly, huge quantities of spent silica gel are generated yearly by laboratories in universities and research centers. Examples of contaminants in industrial spent silica gel include organic solvents, organic compounds, heavy metals, sand, polystyrene and/or polyethylene glycol based resins, vermiculite, thin layers plates, desiccants like sodium and magnesium sulfate, molecular sieves, as well as other adsorbents like alumina, Celite.TM. and Kieselguhr.TM., activated carbon, Florisil.TM., and chemically modified silica gels (C.sub.18, C.sub.8, amino, diol, etc.). Because waste silica is generally disposed of in large bins, it not unusual to find therein foreign objects like gloves, syringes, needles, flasks, magnetic stirrers, labels, filter papers, hand paper, septa, broken glass, cotton, glass wool, chemical product bottles etc. As a result, spent silica gel is classified as a hazardous waste and poses serious environmental problems.
Because of its relatively low cost, little attention has been paid to silica gel regeneration in the past. Also, although large volumes of silica gel are generated each year, the volume per company is generally not sufficiently important to warrant the investment of developing advanced regeneration technologies internally. It is well known that silica is relatively stable in strongly acidic media or when heated at high temperatures. Because of its high temperature stability, most regeneration processes developed in the past proposed a simple heat treatment, alone or in combination with acid washing. Examples of such processes include those disclosed in U.S. Pat. No. 4,676,964; U.S. Pat. No. 4,401,638 and U.S. Pat. No. 4,008,994.
Gas stripping has been used on many adsorbents for regeneration purposes, for example in U.S. Pat. No. 5,227,598; U.S. Pat. No. 5,187,131; U.S. Pat. No. 4,971,606; U.S. Pat. No. 3,917,733; U.S. Pat. No. 4,008,289; U.S. Pat. No. 4,575,566 & U.S. Pat. No. 4,404,118. This process has serious limitations since it is effective inasmuch the contaminants are known. Microwaves are also known to be effective for the desorption of contaminants on adsorbents or simple drying thereof. This technology requires significant capital investment to acquire the equipment, and the electricity requirements are substantial.
Oxidation of organic contaminants on adsorbents has been performed by hydrogen peroxide or hydroxyl radicals generated in situ, for example in U.S. Pat. No. 4,012,321 (H.sub.2 O.sub.2 /UV); U.S. Pat. No. 4,861,484 (H.sub.2 O.sub.2 /catalyst); U.S. Pat. No. 4,261,805 (H.sub.2 O/O.sub.2 /X-rays); U.S. Pat. No. 5,182,030 (H.sub.2 O.sub.2 /light after adsorption of a photoreactor). Again, these processes can be effective only to the extent that the contaminants are known. They are therefore highly specific, and also require complex and costly equipment.
Drying of the final silica gel, whether regenerated under current technologies or freshly prepared, can be done in many ways, direct heating being the most commonly used method. The use of water miscible solvents like alcohols or ketones with further heating to remove residual solvent at temperatures below 100.degree. C. are also known.
All the above processes have their drawbacks. The combination acid treatment-heat treatment produces, after the first acid treatment, a highly contaminated aqueous effluent because of the presence of significant concentrations of degraded organic wastes. In fact, acidic degradation generates highly polar, non-water soluble organic compounds with much greater affinity for silica than for water. The heat treatment, in addition to having high oxygen requirements, thus results in incomplete combustion of the organic compounds, still present in relatively high concentrations even over extended periods of time and after the acid washing. Furthermore, black carbon decomposition products are generated during the process.
The combination organic solvent treatment-heating treatment is ineffective when the silica gel contains inorganic contaminants like heavy metals, because the latter are generally insoluble in organic solvents. The inorganic contaminants will therefore accumulate in the silica overtime. Further, such method requires on-site, specific segregation of used silica gel to determine the nature and extent of the contaminant(s). As stated above, only small volumes can be treated at the same time.
In view of the above, there is therefore a great need to develop a universal process for the regeneration of contaminated particulate materials like silica, silica gel, alumina, clays, silicate materials, sand and the like. Such process should be able to regenerate the materials in a manner such that its properties are at least as good as the virgin materials available on the market, whatever the nature and number of contaminants present originally in the contaminated material.